1. Field of the Invention
The invention relates to an electric parking brake control apparatus that controls an electric parking brake system of a vehicle, an electric parking brake system, and a method for controlling an electric parking brake system.
2. Description of the Related Art
A vehicle brake system is required to serve two functions, that is, a function as a service brake (hereinafter, referred to as “hydraulic brake”) that decelerates or stops a vehicle in motion, and a function as a parking brake that maintains the vehicle at standstill. A brake system that performs two functions using a shared structure has been suggested in order to suppress increases in the number of components and cost. A technology in which the hydraulic pressure of the hydraulic brake is used to apply the parking brake may be employed in order to further simplify the structure to reduce the cost. However, in order to reliably maintain a required braking force even if the hydraulic pressure is gradually decreased while the vehicle is parked or to prevent leakage of hydraulic fluid, a technology in which a tension is applied to a cable so as to push a braking member against a disc or a drum is employed in some parking brakes, as described in, for example, Japanese Patent Application Publication No. JP-2005-280640 (JP-A-2005-280640).
A vehicle brake system described in JP-A-2005-280640 includes a hydraulic brake in which hydraulic pressure can be maintained, and a parking brake that automatically applies a tension to a cable so as to maintain a vehicle at standstill. When determining that an accelerator pedal is fully released and the vehicle speed is zero, the vehicle brake system applies the parking brake while maintaining the hydraulic pressure in the hydraulic brake. With this structure, it is possible to automatically apply the parking brake only when needed. After the parking brake is applied, it is possible to maintain the vehicle at standstill mechanically instead of using the hydraulic pressure.
In a parking brake that is automatically applied, a tension is applied to a cable using driving force supplied from an actuator. A target tension is set with, for example, a road inclination, taken into account. The actuator pulls the cable until the tension detected by a tension sensor reaches the target tension. However, a tension that is lower than the target tension is applied to a wheel in some cases, because the tension detected by the tension sensor is a value from which loss of the tension, which is caused due to friction of the cable and a tension transmission system, has not been subtracted. In the vehicle parking brake system described in JP-A-2005-280640, the hydraulic brake may be either released or kept applied, after the parking brake is applied. However, if the hydraulic brake is released, there is a possibility that the vehicle will start moving due to the lack of tension caused by the loss of tension. On the other hand, if the hydraulic brake is kept applied, there is a possibility that the hydraulic pressure is gradually decreased and the vehicle will start moving.
To prevent the vehicle from moving, a tension that is higher than the target tension may be applied to the cable in anticipation of a decrease in the tension from the target tension or lack of the tension. However, it is necessary to provide an actuator having a higher performance in order to apply such higher tension to the cable. This causes inconveniences such as increases in the cost and size of the actuator.